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Volume 23, Issue 50
September 7, 2017
Pages 11992–12003

Special Issue: Special Issue Celebrating the 150th Anniversary of the Gesellschaft Deutscher Chemiker (German Chemical Society)

Concept

Catalytic NH₃ Synthesis using N₂/H₂ at Molecular Transition Metal Complexes: Concepts for Lead Structure Determination using Computational Chemistry

Authors

Dr. Markus Hölscher^* and Prof. Dr. Walter Leitner

DOI: 10.1002/chem.201604612

 http://onlinelibrary.wiley.com/wol1/doi/10.1002/chem.201604612/abstract

Abstract

While industrial NH₃ synthesis based on the Haber–Bosch-process was invented more than a century ago, there is still no molecular catalyst available which reduces N₂ in the reaction system N₂/H₂ to NH₃. As the many efforts of experimentally working research groups to develop a molecular catalyst for NH₃ synthesis from N₂/H₂ have led to a variety of stoichiometric reductions it seems justified to undertake the attempt of systematizing the various approaches of how the N₂ molecule might be reduced to NH₃ with H₂ at a transition metal complex. In this contribution therefore a variety of intuition-based concepts are presented with the intention to show how the problem can be approached. While no claim for completeness is made, these concepts intend to generate a working plan for future research. Beyond this, it is suggested that these concepts should be evaluated with regard to experimental feasibility by checking barrier heights of single reaction steps and also by computation of whole catalytic cycles employing density functional theory (DFT) calculations. This serves as a tool which extends the empirically driven search process and expands it by computed insights which can be used to rationalize the various challenges which must be met.

Catalytic N2-to-NH3 Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET

 http://pubs.acs.org/doi/full/10.1021/acscentsci.7b00014

Catalytic N₂-to-NH₃ Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET

Matthew J. Chalkley†, Trevor J. Del Castillo†, Benjamin D. Matson†, Joseph P. Roddy, and Jonas C. Peters^*

Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech), Pasadena, California 91125, United States

ACS Cent. Sci., Article ASAP

DOI: 10.1021/acscentsci.7b00014

Publication Date (Web): February 14, 2017

A synthetic Fe complex catalyzes nitrogen fixation at lower driving force increasing its relevance as a functional model of nitrogenase. Theory and experiment lead to the proposal that protonated cobaltocenes may play a role as PCET reagents.

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Abstract

We have recently reported on several Fe catalysts for N₂-to-NH₃ conversion that operate at low temperature (−78 °C) and atmospheric pressure while relying on a very strong reductant (KC₈) and acid ([H(OEt₂)₂][BAr^F₄]). Here we show that our original catalyst system, P₃^BFe, achieves both significantly improved efficiency for NH₃ formation (up to 72% for e^– delivery) and a comparatively high turnover number for a synthetic molecular Fe catalyst (84 equiv of NH₃ per Fe site), when employing a significantly weaker combination of reductant (Cp*₂Co) and acid ([Ph₂NH₂][OTf] or [PhNH₃][OTf]). Relative to the previously reported catalysis, freeze-quench Mössbauer spectroscopy under turnover conditions suggests a change in the rate of key elementary steps; formation of a previously characterized off-path borohydrido–hydrido resting state is also suppressed. Theoretical and experimental studies are presented that highlight the possibility of protonated metallocenes as discrete PCET reagents under the present (and related) catalytic conditions, offering a plausible rationale for the increased efficiency at reduced driving force of this Fe catalyst system.

http://www.nature.com/nature/journal/v526/n7571/full/nature15246.html

Binding of dinitrogen to an iron–sulfur–carbon site

• Ilija Čorić,
• Brandon Q. Mercado,
• Eckhard Bill,
• David J. Vinyard
• & Patrick L. Holland

Nature

526,

96–99

(01 October 2015)

doi:10.1038/nature15246

Received

27 May 2015

Accepted

24 July 2015

Published online

23 September 2015

Nitrogenases are the enzymes by which certain microorganisms convert atmospheric dinitrogen (N₂) to ammonia, thereby providing essential nitrogen atoms for higher organisms. The most common nitrogenases reduce atmospheric N₂ at the FeMo cofactor, a sulfur-rich iron–molybdenum cluster (FeMoco)^{1, 2, 3, 4, 5}. The central iron sites that are coordinated to sulfur and carbon atoms in FeMoco have been proposed to be the substrate binding sites, on the basis of kinetic and spectroscopic studies^{5, 6, 7}. In the resting state, the central iron sites each have bonds to three sulfur atoms and one carbon atom. Addition of electrons to the resting state causes the FeMoco to react with N₂, but the geometry and bonding environment of N₂-bound species remain unknown⁵. Here we describe a synthetic complex with a sulfur-rich coordination sphere that, upon reduction, breaks an Fe–S bond and binds N₂. The product is the first synthetic Fe–N₂ complex in which iron has bonds to sulfur and carbon atoms, providing a model for N₂ coordination in the FeMoco. Our results demonstrate that breaking an Fe–S bond is a chemically reasonable route to N₂ binding in the FeMoco, and show structural and spectroscopic details for weakened N₂ on a sulfur-rich iron site.